1. Field of the Present Invention
The present invention relates to a process for producing inorganic fine grains, to inorganic fine grains, to a rare earth element-activated barium fluorohalide fluorescent substance, and to a radiation image conversion panel.
2. Description of the Related Art
As a fluorescent substance for a radiation image conversion panel which is used in radiography, a divalent europium-activated barium fluorohalide fluorescent substance (BaFX:Eu2+ wherein X is at least one of Cl, Br and I; this is applied hereinafter) has been so far known. This fluorescent substance, when excited with radiation such as X-rays, electron rays or ultraviolet rays, allows near infrared luminescence (instantaneous luminescence) with maximum luminescence near 390 nm.
It has been further found that when this fluorescent substance is irradiated with the radiation and then excited with an electromagnetic wave (excitation light) in a visible to infrared region, it allows near ultraviolet luminescence, namely stimulation luminescence. As described in JP-A No. 55-12145, this fluorescent substance has attracted much interest as a fluorescent substance for a radiation image conversion panel employed in a radiation image conversion method using stimulation of the fluorescent substance. Among others, a divalent europium-activated barium fluoroiodide fluorescent substance (BaFI:Eu2+) has a luminescence wavelength in a long wave side. Accordingly, there has been a proposal that a semiconductor laser beam having an oscillation wavelength in a near ultraviolet region is used as excitation light and this fluorescent substance is employed in combination therewith.
Barium fluoroiodide (BaFI) has been used as the divalent europium-activated barium fluoroiodide fluorescent substance or a raw material for production of barium halide fluorescent substance with divalent europium-activated iodine (so-called a fluorescent substance raw powder).
For obtaining a barium fluorohalide, a method in which at least one compound selected from the group consisting of barium carbonate, barium nitrate and barium sulfate is reacted with at least one compound selected from the group consisting of hydrogen chloride, hydrogen bromide and hydrogen fluoride to form a barium halide and the product is then reacted with hydrogen fluoride to form a barium fluorohalide has been known. However, barium fluoroiodide is, unlike barium fluorobromide, high in solubility in water. Therefore, the mere reaction of these materials in an aqueous medium is problematic in that the product cannot be obtained in satisfactorily high yield and impurities tend to be incorporated.
Further, in consideration of the fact that the resulting crystal of barium fluoroiodide are used in a radiation image conversion panel, crystal in a cubic form having volume-average grain size of not more than 10 xcexcm are preferable. Thus, there is a demand for the development of a process to meet such requirements.
JP-A No. 7-233369 discloses a method in which BaX2 is reacted with an inorganic fluoride (for example, NH4F) to produce a rare earth element-activated barium fluorohalide (BaFX:Ln). JP-A No. 11-29324 discloses the method described in JP-A No. 7-233369 is applied to production of barium fluoroiodide. Crystal of barium fluoroiodide obtained by this method are square (cubic), however, volume-average grain size (Dm) of those is not less than 10 xcexcm. When the crystal is used in the existing radiation image conversion panel, graininess is sometimes decreased.
In the method described in JP-A No. 10-140148, barium fluoroiodide grains having a small volume-average grain size can be produced, but the grain forms are indefinite. Accordingly, scattering of excitation light cannot be controlled in a radiation image conversion panel, which might cause deterioration of an image quality.
Thus, a method for producing inorganic fine grains having a definite form and a small grain size is sometimes required not only in obtaining the foregoing raw grains of the fluorescent substance but also in obtaining general inorganic fine grains.
Under these circumstances, the present invention aims to provide a process for producing inorganic fine grains in a definite form having a small grain size, and inorganic fine grains formed by this process. Further, the present invention aims to provide a rare earth element-activated barium fluorohalide fluorescent substance using at least the inorganic fine grains formed by this process as a raw material, and a radiation image conversion panel with an excellent image quality using the rare earth element-activated barium fluorohalide fluorescent substance.
The foregoing aims are attained by the following approaches.
That is, a first aspect of a process for producing inorganic fine grains in the present invention is a process for producing inorganic fine grains, which comprises adding, to a solution containing at least one inorganic compound, a solid matter substantially insoluble in the solution, subjecting the solution to a procedure of promoting crystallization or precipitation to form crystal or precipitate, and separating the resulting crystal or precipitate.
A second aspect of the process for producing the inorganic fine grains in the present invention is the process for producing the inorganic fine grains according to the first aspect, in which the resulting crystal or precipitate is barium fluorohalide.
A third aspect of the process for producing the inorganic fine grains in the present invention is the process for producing the inorganic fine grains according to the first aspect, in which the solution is a mixture of a BaI2 aqueous solution containing at least one rare earth element and a fluoride aqueous solution, a Ba concentration is not more than 3.0 mol/liter and a F/Ba molar ratio is not more than 1.
A fourth aspect of the process for producing the inorganic fine grains in the present invention is the process for producing the inorganic fine grains according to the third embodiment, in which the fluoride aqueous solution is NH4F aqueous solution.
A first aspect of inorganic fine grains in the present invention is inorganic fine grains produced by adding, to a solution containing at least one inorganic compound, a solid matter substantially insoluble in the solution, subjecting the solution to a procedure of promoting crystallization or precipitation to form crystal or precipitate, and separating the resulting crystal or precipitate, in which the solution is a mixture of BaI2 aqueous solution containing at least one rare earth element and a fluoride aqueous solution, a Ba concentration is not more than 3.0 mol/liter and a F/Ba molar ratio is not more than 1, the inorganic fine grains being represented by the following basic composition formula (I), having a hexahedral form and having a volume-average grain size of 1 to 10 xcexcm.
BaFI:xLnxe2x80x83xe2x80x83(I) 
wherein
Ln represents at least one of Ce, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er, Tm and Yb, and
x represents a value of 0 less than xxe2x89xa60.2.
A second aspect of the inorganic fine grains in the present invention is the inorganic fine grains according to the first aspect, in which the fluoride aqueous solution is an NH4F aqueous solution.
A third aspect of the inorganic fine grains in the present invention is the inorganic fine grains according to the first aspect, in which an aspect ratio is 0.5 to 2.
A first aspect of a rare earth element-activated barium fluorohalide fluorescent substance in the present invention is a rare earth element-activated barium fluorohalide fluorescent substance which is produced using at least the inorganic fine grains according to the first aspect thereof.
A first aspect of a radiation image conversion panel in the present invention is a radiation image conversion panel in which a rare earth element-activated barium fluorohalide fluorescent substance produced using the inorganic fine grains according to the first aspect thereof is contained in a fluorescent substance layer.